For many end-use applications, there are few or no polymers available possessing the requisite properties to satisfy the applications for which they are needed. Furthermore, even when a polymer having suitable characteristics is known, it is sometimes not feasible to produce the polymer, due to the expense or the unavailability of the necessary raw materials, or because of the costs entailed in the synthesis. In such instances, resort is sometimes had to the preparation of blends of polymers, each of which contributes its individual characteristics to the whole, thus providing composite characteristics that meet the demands of the application.
Some polymer blends, for instance, have enjoyed widespread use due to their combined properties, for example, high-impact polystyrene and acrylonitrile/butadiene/styrene resins. In such systems, the desirable mechanical properties achieved are associated with graft-producing chemical reactions occurring at the interface of the polymer phases.
Unfortunately, the blending of polymers with suitable properties is frequently difficult to achieve due to the mutual incompatibility of the polymers. This results in immiscibility of the polymers and prevents uniformly dispersed polymer blends from being obtained. By way of example, polar polymers are substantially immiscible in non-polar polymers, making their blending difficult under normal circumstances. In this connection, the instability of the polymer blend phase morphology is believed to be a consequence of interfacial tension between the polymer phases, and in the absence of phase-behavioral modifying agents, the morphology of the polymer mixtures is to a substantial degree dependent upon the thermal and mechanical treatment to which the polymer mixtures have previously been subjected.
Polymer incompatibility has long been identified as a problem, and various methods have been proposed for circumventing it. For example, it has been recognized for some time that the incorporation of relatively small amounts of additional polymers, for instance, graft or block polymers, sometimes referred to as "compatibilizing agents", to binary blends of otherwise immiscible polymers tends to stabilize the phase morphology of the mixtures. In this regard, polystyrene/polyethylene graft and block copolymers have been used to make polyethylene/polystyrene blends. Similarly, polypropylene maleic anhydride graft copolymers have been employed to achieve satisfactory combinations of polypropylene and nylon-6, and polyethylene/polystyrene block copolymers have been added to polycarbonate/polyethylene melt blends. While such compatibilizing agents have been used in connection with the polymer mixtures described, the preparation of certain other desirable mixtures has previously been difficult or impossible to achieve, due to the fact that efficient compatibilization agents have up to now been unavailable.